The present invention relates to a method for the preparation of an organopolysiloxane or, more particularly, to a method for the preparation of an organopolysiloxane having tetrafunctional siloxane units and containing little amount of residual alkoxy groups so as to be useful as a reinforcing agent of silicone rubbers.
Among various types of organopolysiloxanes, those soluble in organic solvents and consisting of monofunctional organosiloxy units represented by the general unit formula R.sub.3 SiO.sub.0.5, in which R is a hydrogen atom or a monovalent hydrocarbon group, referred to as the M units hereinbelow, and tetrafunctional siloxane units of the formula SiO.sub.2, referred to as the Q units hereinbelow, are widely used in the art of silicone products, for example, as a reinforcing agent of silicone rubbers. Such an organopolysiloxane consisting of the M units and Q units is prepared, for example, by the method disclosed in U.S. Pat. Nos. 2,676,182 and 2,814,601, in which a water-soluble basic silicate such as sodium orthosilicate is converted into a silicic acid oligomer by the addition of hydrochloric acid or sulfuric acid followed by the reaction with a trialkyl chlorosilane.
This method, however, has several problems as an industrial process because, since the silicic acid oligomer is relatively unstable, difficulties are encountered in the control of the molecular weight distribution in the organopolysiloxane product so that the molar ratio of the M units and the Q units in the organopolysiloxane product cannot be always consistent with the target molar ratio. In addition, since the waste water coming from the process necessarily contains a large amount of the waste acid used for the neutralization of the starting basic silicate as well as the hydrochloric acid produced as a by-product in the reaction of the trialkyl chlorosilane and also a large amount of an alcohol admixed with the reaction mixture with an object of stabilization of the reaction mixture, a large cost is required for the disposal of the waste water in order not to cause the problem of environmental pollution. When the waste water contains hydrochloric acid in a substantial concentration, in particular, the hydrogen chloride gas emitted therefrom is very harmful against human health with a strong irritating odor and strong corrosion is unavoidable on the apparatuses and pipe lines as well as other auxiliary instruments in the manufacturing plant so that they must be constructed by using highly corrosion-resistant but very expensive materials in addition to the disadvantage of large man power and very high cost required for the maintenance of the plant.
Alternatively, the organopolysiloxane of this type can be prepared by the method taught in U.S. Pat. No. 2,857,356, in which an alkyl silicate and a trialkyl chlorosilane are subjected to cohydrolysis in the presence of hydrochloric acid, or by the method taught in Japanese Patent Kokai No. 61-195129, in which an alkyl silicate or a partial hydrolysis product thereof is added dropwise to a trialkyl chlorosilane in the presence of hydrochloric acid. As compared with the first described method, these methods have an advantage because the molar ratio of the M units and the Q units or the molecular weight distribution in the organopolysiloxane product can be controlled relatively easily while they have disadvantages that measures for the disposal of waste water and against the adverse effects of toxic and corrosive hydrochloric acid must be undertaken likewise as in the above described method since a large amount of hydrochloric acid must be added to the reaction mixture in addition to the hydrochloric acid produced by the reaction or the alcohol admixed in the reaction mixture.
At any rate, these known methods are each industrially disadvantageous because a large volume of acid-containing waste water must be safely disposed and the productivity is low with a relatively low yield of the organopolysiloxane product per unit reaction volume as a consequence of the use of a large volume of organic solvents as a hydrolysis aid. When an organopolysiloxane containing a large amount of the Q units or, in particular, having the molar ratio of the M units to the Q units not exceeding 2 is desired, it is a rather difficult matter to adequately control the reaction so as to obtain an organopolysiloxane having the molar ratio of the units and molecular weight distribution exactly controlled as desired. Furthermore, difficulties are encountered in the preparation of an organopolysiloxane having a controlled amount of the silanol groups or alkoxy groups with good reproducibility.
As a measure to dissolve the above described disadvantages, accordingly, a method is proposed in Japanese Patent Kokai No. 63-256628, in which an organosilane or an organosiloxane is reacted with an alkyl silicate or a partial hydrolysis product thereof in the presence of a sulfonic acid group-containing compound and/or phosphonitrile chloride as a catalyst. This method has advantages that control of the molar ratio of the M units to the Q units is relatively easy and no hydrochloric acid is produced as a by-product of the reaction. This method, however, is not free from the disadvantages that, since the reaction is conducted under atmospheric pressure, residual amount of the alkoxy groups must be removed by the addition of an excess amount of water taking a long reaction time during which gelled materials are sometimes formed in the reaction mixture resulting in very poor filtrability of the mixture with a greatly decreased productivity. When such a measure is not undertaken to leave a large amount of the residual alkoxy groups in the product, the applicability of such an organopolysiloxane product as a reinforcing agent of silicone rubbers would be low because no sufficient reinforcing effect can be obtained therewith and the mechanical strengths of the silicone rubber compounded with such an organopolysiloxane would be subject to gradual degradation in the lapse of time.